From what I have read in these forums between production of 1999 and 2000 ceramic magnets replaced the alnico ones, then I believe after 2002 the alnico's were reintroduced to the StingRay whilst ceramic magnets were fitted only to the Sterling basses.
The best place for reference is Rod Trussbroken's website at this link:
Sterlings (both 4 and 5 string) - have both always ceramic
Stingray4 - has always been alnico
Stingray5- alnico from 1987-1991, then ceramic from 1991 to 2008, and alnico from 2008 to current